Tensioner with expanding spring for radial frictional asymmetric damping

ABSTRACT

A tensioner is disclosed that may be part of a power system where the tensioner provides tension to an endless power transmitting element such as a belt, chain, or other continuous loop. The tensioner has an arm that is rotatable about a first axis and includes an arm arbor having a slot therethrough, a bushing having a protrusion and being positioned adjacent the arm arbor with the protrusion received in the arm arbor&#39;s slot, and a spring coupled to the arm urging the arm to rotate about the first axis into tensioning engagement with a power transmitting element. The spring is positioned where it can radially expand into contact with the protrusion of the bushing as the arm is rotated in a direction opposite the direction of tensioning engagement such that the bushing is urged radially outward relative to the arm arbor to provide frictional damping.

TECHNICAL FIELD

The present invention relates generally to tensioners and moreparticularly to an asymmetrically damped tensioner utilizing anexpanding spring to provide radial friction-damping.

BACKGROUND

It is common for a tensioner such as a belt tensioner to have a means todampen movement of the tensioner arm caused by belt tension fluctuation.The required magnitude of this damping depends on many drive factorsincluding geometry, accessory loads, accessory inertia, engine dutycycle and others. For instance, drive systems that have higher torsionalinput or certain transient dynamic conditions may require higher dampingto sufficiently control tensioner movement. Although higher damping isvery effective at controlling arm movement, it can also be detrimentalto other critical tensioner functions (e.g. slow or no response to slackbelt conditions). In addition, variation or change in damping that occuras a result of manufacturing variation, operating temperature andcomponent break-in or wear can also cause the tensioner to beunresponsive.

Timing belt systems have benefited from the use of asymmetric damping toaddress this problem. An asymmetrically damped tensioner providesdamping when additional belt tension is encountered, but is free torespond to slack belt conditions. Although asymmetric functionality maynot be required for all other front end accessory drive tensioners, thepotential for increased service life, solving other transient dynamicsystem problems including belt slip, or simply making the tensioner lesssensitive to damping variation make it a desirable design option.

Many belt tensioner damping mechanisms that utilize frictional dampinguse axial forces to move components of the tensioner to create thefrictional force that does the damping. These designs tend to require ameans to contain the axial force and some components of the belttensioner must be more robust to withstand the axial force over thelifetime of the tensioner.

SUMMARY

One aspect of the disclosed tensioners is a tensioner embodiment wherethe radial damping force can be contained within a support wall ratherthan relying on joints. The radial damping is preferably asymmetric.

In one embodiment, a tensioner is disclosed that may be part of a powersystem where the tensioner provides tension to an endless powertransmitting element such as a belt, chain, or other continuous loop.The tensioner has an arm that is rotatable about a first axis andincludes an arm arbor having a slot therethrough, a bushing having aprotrusion and being positioned adjacent the arm arbor with theprotrusion received in the arm arbor's slot, and a spring coupled to thearm urging the arm to rotate about the first axis into tensioningengagement with a power transmitting element. The spring is positionedwhere it can radially expand into contact with the protrusion of thebushing as the arm is rotated in a direction opposite the direction oftensioning engagement such that the bushing is urged radially outwardrelative to the arm arbor to provide frictional damping.

In another embodiment, the tensioner includes a support member housingthe spring, the arm arbor, and the bushing with the bushing adjacent thesupport member and the arm arbor between the spring and the bushing.Accordingly, when the spring is expanded radially it urges the bushinginto frictional engagement with the support member to provide thefrictional damping.

The bushing may include a longitudinal slit therethrough that allowsradial expansion of the bushing in response to the radially expansion ofthe spring. In one embodiment, the bushing includes a substantiallycylindrical sleeve that has the longitudinal slit therein and has atleast one protrusion on its inner surface. The bushing may also have aflange extending outward from one end of its sleeve.

The arm arbor of the arm preferably has a fixed diameter such that thearm arbor does not respond to the radial expansion of the spring.Instead, just the bushing is expanded radially by the expanding spring.The tensioner may also include a cap enclosing the spring within thetensioner.

In one embodiment, the arm includes a pulley rotatably mounted about asecond axis, the second axis being spaced from and parallel to the firstaxis.

In another embodiment, a tensioner is disclosed that may be part of apower system where the tensioner provides tension to an endless powertransmitting element. The tensioner includes a support member comprisinga shaft defining a first axis, an arm comprising an arm arbor mounted onthe shaft for rotatable movement of the arm about the first axis. Thearm arbor defines a cavity that has at least one slot opening therein.The tensioner also includes a bushing between the support member and thearm. The bushing includes a protrusion, which is received in the slot ofthe arm arbor. Additionally, the tensioner has a spring received in thecavity of the arm arbor and coupled to the arm. The spring urges orbiases the arm to rotate about the first axis into tensioning engagementwith a power transmitting element. The spring is also positioned toradially expand into contact with the protrusion of the bushing as thearm is rotated in a direction opposite the direction of tensioningengagement such that the bushing is urged radially outward relative tothe arm arbor into frictional engagement with the support member toprovide frictional damping.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a front view of an engine which utilizes an embodiment of atensioner.

FIG. 2 is an exploded perspective view of an embodiment of a tensioner.

FIG. 3 is a side, cross-sectional view of the tensioner of FIG. 1 takenalong line 3-3.

FIG. 4 is a cross-sectional view of the tensioner of FIG. 3 taken alongline 4-4.

FIG. 5 is a cross-sectional view of an embodiment of a tensioner showingthe underside of the cap connected to the arm, pivot shaft, and spring.

FIG. 6 is a side, bottom perspective view of the cap of FIG. 5.

DETAILED DESCRIPTION

The following detailed description will illustrate the generalprinciples of the invention, examples of which are additionallyillustrated in the accompanying drawings. In the drawings, likereference numbers indicate identical or functionally similar elements.

The damping mechanism and tensioner disclosed herein provide anasymmetric frictional damper. The tensioner is typically part of a powersystem where the tensioner provides tension to an endless powertransmitting element such as a belt, chain, or other continuous loopthat are in a system driven by at least one source and that may alsodrive an accessory. The power transmitting element and the tensioneroperate in concert with the tensioner providing tension to the endlesspower transmitting element as needed and responding to dynamicconditions thereof.

Referring now to FIG. 1, an engine is generally indicated by thereference numeral 20 and utilizes an endless power transmitting element21 for driving a plurality of driven accessories as is well known in theart. The belt tensioner of this invention, generally designated as 100,is utilized to provide a tensioning force on the endless powertransmitting element 21. The endless power transmission element 21 maybe of any suitable type known in the art. The tensioner 100 isconfigured to be fixed to a mounting bracket or support structure 24 ofthe engine 20 by a plurality of fasteners 25. The fasteners may bebolts, screws, welds, or any other suitable fastener known in the artthat will hold the tensioner in place during operation of the engine.The mounting bracket or supporting structure 24 may be of anyconfiguration and include any number of openings for receiving thefasteners 25.

Tensioning a slack endless power transmitting element with the tensionerdisclosed herein is unusual in that it is the winding of an unwoundspring that operates to rotate the arm of the tensioner to providetension, which will be referred to herein as the tensioning direction T.In the opposite direction, referred to herein as the winding directionW, the tensioner arm may be considered to be winding in response to aprevailing force of the endless power transmitting element which istightening in the span where the tensioner resides; however,uncharacteristically for tensioners, the winding of the tensioner armcorresponds to an unwinding of the spring within the disclosedtensioners.

The winding of the tensioner may have some potentially undesirableeffects upon the drive system's intended function. To mitigate theseundesirable effects it may be helpful to have a damper or dampingmechanism, for example a frictional damper, incorporated in thetensioner to resist the movement of the power transmitting element,without adversely affecting rotation of the tensioner, in particular itsarm to tension the power transmitting element. This kind of frictionaldamping is generally known as asymmetric damping, and in the tensionersdisclosed herein the unwinding of the spring provides such damping. Theunwinding of the spring expands its coils outward, enlarging its coildiameter, which is herein utilized to provide the asymmetric frictiondamping by having the spring act upon another component of the tensionerin that the spring urges into frictional engagement with anothersurface.

Referring to FIGS. 2-3, the tensioner 100 provides asymmetric frictionaldamping to the movement of an arm 102 through the expansion of spring106 as it is unwound in response to a belt load or other prevailingforce of the endless power transmitting element which is tightening inthe span where the tensioner resides. The spring 106 transfers anoutwardly directed force, a radial force, from its expanding coils to abushing 108 to urge the bushing 108 into frictional engagement with aninterior surface 146 of a support member 114 that houses at least partof the spring 106 and bushing 108 such that substantial frictionaldamping is applied to the belt tensioner in the winding direction W. Asexplained above, the winding direction occurs when increasing tensioncauses the endless power transmitting element to lift the tensioner armin a direction away therefrom. The tensioner resists rotating in thewinding direction W with a frictional damping force, but does notsubstantially resist movement of the tensioner arm toward the belt withthe same frictional damping force.

Unique to the construction of the tensioners disclosed herein is the useof the radially expanding spring where the radial expansion provides theforce to urge parts into frictional engagement to provide damping andthe radially expanded, i.e., unwound, spring then applies a torsionalforce to apply torque to the tensioner arm to rotate the tensioner armin the tensioning direction T, i.e., toward the power transmittingelement.

The tensioner's application of radial force, rather than axial force,allows some of the components to be made from less costly materials asthe components and joints do not need to be as robust as they would towithstand axial forces. The absence of axial forces allows somecomponents to be made thinner, which can reduce the weight of thetensioner and the cost. Any radial forces that exist in the tensionercan be contained effortlessly within the support member of the belttensioner.

The tensioner 100 includes a tensioner arm 102 rotatable about a firstaxis A in the tensioning direction T and in the winding direction Wopposite the tensioning direction as shown in FIG. 3, a spring 106, abushing 108, a support member 114, and a cap 118. The arm 102 includes apulley 120 rotatably mounted to its first end 130 for rotation about asecond axis B that is spaced from and parallel to the first axis A. Thepulley 120 may be coupled to the arm 102 with a pulley bolt 122 or otherfastener and may include a dust cover 124.

The arm 102 includes, at its second end 132, an arm arbor 104 extendingfrom the arm about the first axis A. The arm arbor 104 may include asleeve 152 that has an open first end 154 and a partial bottom 117 thatdefines an open second end 156 that has a smaller opening compared tothe first end 154. In one embodiment, the sleeve 152 is generallycylindrical and defines a housing 150 that may receive the spring 106.Within the sleeve 152 one or more slots 116 are present that extendtherethrough, i.e., the slots are open from the exterior surface of thearm arbor 104 into its interior. Upon assembly, the first end 154 of thesleeve 152 may be closed by the cap 118 and the second end 156 may beclosed by the support member 114. The cap 118 and support member 114 mayenclose the other components of the tensioner, for example, the spring106, the arm arbor 104, and the bushing 108, and protect them fromcontaminants.

In one embodiment, the arm arbor 104 includes two slots 116, morepreferably as shown in FIG. 2, three slots 116, but is not limited toany particular number of slots. The slots 116 may be positioned equallydistant apart about the arm arbor 114, which is advantageous todistribute the force exerted by the expanding spring 106 more uniformlyonto the bushing 108. In one embodiment, the slots 116 may extendthrough the sleeve 152. The slots 116 may be any shape and/orconfiguration that allows the protrusions 110 of the bushing to extendinto the cavity 143 defined by the sleeve 152 for contact with spring106 as it expands.

As best seen in FIG. 3, the slots 116 may extend through the sleeve 152and into the partial bottom 117. The portion of the slots 116 in thepartial bottom 117 only extend partially radially, inward into thepartial bottom 117, such that the partial bottom 117 iscircumferentially discontinuous at its outer periphery andcircumferentially continuous at its inner periphery. The inner peripherybeing the edge closest to the first axis A. The circumferentiallycontinuous inner periphery helps stabilize or provide rigidity to theopen second end 156 of the sleeve 152 and provides the arm arbor 114with fixed dimensions. In one embodiment, the sleeve 152 issubstantially cylindrical and has a fixed diameter.

The partial bottom 117, as best seen in FIG. 4, includes an abutmentfeature 180 positioned within the interior of the sleeve 152. Theabutment feature 180 receives the first end 107 of the spring 106.Accordingly, when the arm arbor 104 rotates with the arm 102, theabutment feature 180 urges the spring 106 to unwind and radially expandits diameter. In one embodiment, the abutment feature 180 is a partitionor protrusion that provides a generally planar surface for a generallyflat cut end of the spring 106 to abut thereagainst in direct contact.In another embodiment, the abutment feature 180 may be a sleeve, abracket, a recess, or other receptacle that the spring end 107 fits intoto connect the spring to the arm arbor 104 for movement therewith.

In one embodiment, the abutment feature 180 may be a ramping feature,which depending on the ramp direction, could either increase or decreasethe outward expansion of the spring. One of skill in the art willappreciate that the shape and/or contour of the abutment feature 180 maybe such that the tensioner could have asymmetric or progressive damping.

The second end 132 of the arm 102 may also include a flange 158 aboutthe periphery where the arm arbor 104 connects to the arm 102. Theflange 158, upon assembly of the tensioner 100, may seat upon flange 115of the support member 114. Extending from flange 158 there may be a tab140 projecting outward that may act as a stop to limits the rotationalmovement of the arm 102 about first axis A when the tab 140 contacts astop, for example, stop 142 on the support member 114 and/or tab 136 onthe cap 118.

The arm arbor 104 is received in the cavity 143 of the support member114. The support member 114 has a closed end 160 and an open end 162 andincludes a pivot shaft 144 that extends from the closed end 160 into thecavity 143 and about which the arm arbor 104 rotates. The support member114 may facilitate mounting the tensioner 100 in place relative to anendless power transmitting element. In one embodiment, the pivot shaft144 is generally centrally positioned within the cavity 143 and has anaxially extending opening 145 or bore that may receive a bolt, screw,pin, or other fastener 25′ (shown in FIG. 1) to hold the assembled belttensioner together and/or to mount the tensioner to a surface relativeto an endless power transmitting element. The support member 114 mayalso receive and/or house at least part of the bushing 108 and spring106.

In one embodiment, the support member 114 may include an upper rim 115or flange extending outward about the periphery of the open end 162 ofthe cavity 143 and a stop 142 projecting outward from the exterior wallthereof proximate to the open end 162 or as an extension of the flange115. In one embodiment, the support member 114 may also include apositioning pin 147 on the exterior surface of the closed end 160 of thecavity 143 that is receivable in a receptacle that may be provided onthe mounting bracket or supporting structure 24 of the engine 20.

As shown in FIGS. 2-3, a bushing 108 is positioned or positionablebetween the arm arbor 104 and the interior surface 146 of the supportmember 114 and is adjacent the exterior surface of the arm arbor 104.The bushing 108 includes a sleeve 119 having a first open end 170 and asecond open end 172 and one or more protrusions 110 extending from thesleeve's interior surface 168 toward the first axis A. In oneembodiment, the sleeve 119 is generally cylindrical. The number ofprotrusions 110 preferably matches the number of slots 116 in the armarbor 104 such that the bushing 108 is mateable with the arm arbor 104with its protrusions 110 received in the slots 116. Accordingly, theprotrusions 110 are shaped to mate with the slots 116 of the arm arbor104. The protrusions 110 are also dimensioned such that they extendthrough the arm arbor 104 into its interior cavity 143 and areaccessible to or by the spring 106 as it expands upon unwinding.

The bushing 108 may also include a flange 113 extending outward from oneend of the sleeve 119, for example, from the first open end 170. In theembodiment of FIGS. 2-3, the bushing 108 includes a slit 112therethrough extending from the first open end 170 to the second openend 172. The slit 112 enables the bushing 108 to expand radially inresponse to the expansion of the spring 106 as it unwinds. In analternate embodiment, the bushing 108 may be generally elastic.

Spring 106 is seated within cavity 143 of the support member 114 withits coils juxtaposed to the protrusions 110 of the bushing 108.Accordingly, when the arm 102 rotates in response to belt loading orother prevailing force of the endless power transmitting element whichis tightening in the span where the tensioner resides, the spring 106will unwind, increasing the coil diameter, and radially expand its coilsinto the protrusions 110 of the bushing 108 thereby directing thebushing 108 radially outward relative to the arm arbor 104, whichremains stationary, and into frictional engagement with the interiorsurface of the support member 114. When the belt loading or otherprevailing force of the power transmitting element dissipates, thetorque built up in the spring 106 as a result of its unwound state urgesthe tensioner arm 102 to rotate in the tensioning direction T as thespring returns to its wound state. Accordingly, the spring 106 iscoupled to the tensioner arm 102 such that the spring provides thetorque to urge the tensioner arm in the tensioning direction T.

The spring 106 is a torsional spring of any shape and/or configuration.In one embodiment, the torsional spring is a round-wire spring. Inanother embodiment, the torsional spring may be a square or rectangularspring or a square or rectangular coil spring. In another embodiment,the torsional spring is a flatwire spring. One of skill in the art willappreciate that to these various torsional springs may require alternatespring end engagement points within the tensioner to provide secureattachments so that the spring winds and unwinds appropriately to biasthe arm.

The spring 106 preferably has a first end 107 coupling the spring 106 tothe tensioner arm 102, in particular to the arm arbor 104, and a secondend 109 coupling the spring 106 to the cap 118. The first end 107 ofspring 106, as discussed above, abuts against or is received in a firstabutment feature 180 of the tensioner arm 102, best seen in FIG. 4, tocouple the tensioner arm 102 to the spring 106 so that rotation of thetensioner arm 102 in the winding direction W unwinds the spring andthereby radially expands the diameter of the spring's coils. Thereafter,the torque of the unwound expanded spring 106 can rotate the tensionerarm 102 in the tensioning direction T to tension a power transmittingelement when the force lifting the tensioner arm in the windingdirection W is reduced. As the spring 106 uses its torque to rotate thearm 102, the spring 106 winds back toward its original position therebyreducing and/or removing the radial force from the protrusions 110 ofthe bushing 108 such that reduced or substantially no frictional dampingto resist rotation of the tensioner arm toward the belt occurs. Thedamping of the tensioner 100 is asymmetric.

The second end 109 of spring 106 is likewise abutted against or receivedin a second abutment feature (item 182 in FIG. 5) located in the cap118. The second abutment feature in the cap 118 may be the same as ordifferent from the first abutment feature 180. It is preferable that thesecond end 109 of the spring is stationary, i.e., held stationary by thecap 118, which is stationary relative to the arm 102. Accordingly, thesecond abutment feature in the cap 118 should be configured to hold thesecond end 109 of the spring 106 stationary.

The cap 118 of FIGS. 1-3 includes a generally centrally located bore 134for receiving a fastener 25′ such as a bolt, screw, rivet, or otherfastener for securing the cap to the tensioner. The bore 134 may becountersunk into the upper surface 135 of the cap to receive the head ofthe fastener. The cap 118 may also include a tab 136 extending outwardtherefrom. The tab 136 may be L-shaped and comprise an arm 138 extendinggenerally horizontally outward from the outer periphery of the cap 118and a flange 139 generally extending vertically down from the end of thearm 138 opposite the periphery of the cap. On the underside 137 of thecap, a second abutment feature for receiving one end of the spring 106may be formed therein or thereon. A track 192 may be recessed into theunderside 137 of the cap for receiving the spring 106 and may define atleast part of the abutment feature and extend away therefrom. The track192 preferably matches the curvature or shape of the spring 106. In oneembodiment, the cap 118 may include more than one tab 136 and the tabsmay fix the cap 118 to the arm 102 and/or the support member 114.

In another embodiment, illustrated in FIGS. 5-6, the cap, generallydesignated as 118′, has a splined attachment to the pivot shaft 144. Thepivot shaft 144 has splined end 186 opposite the pivot shaft's junctionto the closed end 160 of the cavity 143 and a bore 145. The splined end186 provides a mating connection between the support member 114 and cap118′. To mate with the splined end 186, the cap 118′ has a knob 188comprising an internal configuration of alternating ridges 194 andrecesses 196. The cap 118′ is held stationary by the knob's 188connection to the splined end 186 of the pivot shaft 144.

The cap 118′ may include a generally centrally located bore 134′ that ispositioned through the center of the knob 188. The cap 118′ may alsoinclude a track 192′ recessed into the underside 137′ thereof. The track192′ is shaped to match the shape of the torsional spring 106, inparticular, the portion of the spring that includes the second end 109of the spring 106 and at least part of the first coil extendingtherefrom. The track 192′ may also define part of the abutment feature182 against which the cut end of the second end 109 of the spring is indirection contact therewith. The track 192′ may have a protrusion 190extending therein proximal the second end 109 of the spring 106 to aidein maintaining the second end 109 in place in the cap.

The second abutment feature 182 may be similar to that described above.

The embodiments of this invention shown in the drawing and describedabove are exemplary of numerous embodiments that may be made within thescope of the appended claims. It is contemplated that numerous otherconfigurations of the tensioner may be created taking advantage of thedisclosed approach. In short, it is the applicant's intention that thescope of the patent issuing herefrom will be limited only by the scopeof the appended claims.

What is claimed is:
 1. A tensioner comprising: an arm rotatable about afirst axis, the arm comprising an arm arbor having a slot through aportion thereof; a bushing having a protrusion and being positionedadjacent the arm arbor with the protrusion received in the slot of thearm arbor; a spring coupled to the arm urging the arm to rotate aboutthe first axis into tensioning engagement with an endless powertransmitting element, the spring being positioned to radially expandinto contact with the protrusion of the bushing as the arm is rotated ina direction opposite the direction of tensioning engagement such thatthe bushing is urged radially outward relative to the arm arbor toprovide frictional damping.
 2. The tensioner of claim 1 wherein thebushing includes a longitudinal slit therethrough that allows radialexpansion thereof.
 3. The tensioner of claim 2 wherein the bushingcomprises a sleeve that includes the slit and the protrusion andcomprises a flange extending outward from one end of the sleeve.
 4. Thetensioner of claim 3 wherein the sleeve of the bushing is substantiallycylindrical.
 5. The tensioner of claim 2 wherein the arm arbor has afixed diameter.
 6. The tensioner of claim 1 wherein the arm includes apulley rotatably mounted about a second axis, the second axis beingspaced from and parallel to the first axis.
 7. The tensioner of claim 1further comprising a support member housing the spring, the arm arbor,and the bushing with the bushing adjacent the support member and the armarbor between the spring and the bushing.
 8. The tensioner of claim 7wherein the radial expansion of the spring urges the bushing intofrictional engagement with the support member to provide the frictionaldamping.
 9. The tensioner of claim 7 wherein the support member isstationary and includes a shaft that defines the first axis, wherein thearm is rotatably mounted to the shaft.
 10. The tensioner of claim 1further comprising a cap enclosing the spring within the tensioner. 11.The tensioner of claim 11 wherein the spring has a first end coupled tothe arm and a second end coupled to the cap.
 12. The tensioner of claim1 wherein the tensioner provides asymmetric damping.
 13. A tensionercomprising: a support member comprising a shaft defining a first axis;an arm comprising an arm arbor mounted on said shaft for rotatablemovement of the arm about the first axis, the arm arbor defining acavity and having a slot open into the cavity; a bushing having aprotrusion receivable in the slot of the arm arbor, wherein the bushingis between the support member and the arm with the protrusion receivedin the slot; a spring received in the cavity of the arm arbor andcoupled to the arm urging the arm to rotate about the first axis intotensioning engagement with a power transmitting element, the springbeing positioned to radially expand into contact with the protrusion ofthe bushing as the arm is rotated in a direction opposite the directionof tensioning engagement such that the bushing is urged radially outwardrelative to the arm arbor into frictional engagement with the supportmember to provide frictional damping.
 14. The tensioner of claim 13wherein the bushing includes a longitudinal slit therethrough thatallows radial expansion thereof.
 15. The tensioner of claim 14 whereinthe bushing comprises a sleeve that includes the slit and the protrusionand comprises a flange extending outward from one end of the sleeve. 16.The tensioner of claim 15 wherein the sleeve of the bushing issubstantially cylindrical.
 17. The tensioner of claim 13 wherein the armarbor has a fixed diameter.
 18. The tensioner of claim 13 wherein thearm includes a pulley rotatably mounted about a second axis, the secondaxis being spaced from and parallel to the first axis.
 19. The tensionerof claim 13 further comprising a cap enclosing the spring within thetensioner.
 20. The tensioner of claim 19 wherein the spring has a firstend coupled to the arm and a second end coupled to the cap.
 21. Thetensioner of claim 13 wherein the protrusion is dimensioned to extendinto the cavity of the arm arbor.
 22. The tensioner of claim 13 whereinthe arm arbor comprises a generally cylindrical sleeve having an openfirst end and a partial bottom that defines an open second end that hasa smaller opening compared to the first end.
 23. The tensioner of claim22 wherein the slot extends through the sleeve and into the partialbottom such that the bushing may be slide onto the arm arbor.
 24. Thetensioner of claim 13 wherein the tensioner provides asymmetric damping.